A key Senate committee postponed a vote Wednesday on President Trump’s pick to lead the main agency handling immigration enforcement as a coalition of unions raised “serious concern” about Ronald D. Vitiello’s ability to effectively oversee the agency.

The delay comes at a time when the nation is facing a crisis on the southern border and Trump is pressuring agencies and Congress on an immigration crackdown. The timing is also critical because all nominations will expire at the end of the year if the Senate does not act on them.

Sen. Ron Johnson (R-Wis.), chairman of the Senate Homeland Security and Governmental Affairs Committee, said the vote on Vitiello was being delayed so that senators could practice “due diligence” regarding the concerns raised by unions representing Immigration and Customs Enforcement personnel. One issue they mentioned in a letter to the committee is that Vitiello apparently shared an image of Trump on Twitter that compared the president to the cartoon character Dennis the Menace.

Gizmodo reported in February 2017 that in March 2016, Vitiello tweeted a side-by-side comparison of Dennis the Menace and Trump, who was then a presidential candidate, with the words: “This I can tell you! 100%.” Vitiello was the Border Patrol chief at the time.

The account, @VitielloRonald, is locked from public view, and Twitter has not verified that it belongs to Vitiello. A Senate committee spokesman said Vitiello’s tweets are not part of what the committee will examine in its extended review.

His social media activity didn’t go unnoticed at his confirmation hearing. In 2015, Vitiello tweeted a comparison of Democrats to the Ku Klux Klan. He later told senators that it was a “mistake” and a “momentary lapse of judgment.”

Vitiello is a former Border Patrol agent who rose to hold high-ranking positions at Customs and Border Protection, including acting deputy commissioner of CBP after Trump took office in 2017. Vitiello was named in late June as the acting director of ICE and was nominated for the permanent post in August after Thomas D. Homan, Trump’s prior nominee, languished for months and eventually withdrew.

The Department of Homeland Security said in a statement: “It’s not unusual that some members of the Committee would use a pending vote to seek additional information or concessions from the Department or the Administration. We will address the concerns of an individual Senator and pursue an immediate confirmation.”

In a tweet Thursday morning, White House press secretary Sarah Sanders said, “The White House is confident that Deputy Director Vitiello will be confirmed by the Senate quickly.” She added: “During a time when we have an exploding immigration crisis — including a violent Caravan — on our Southern Border, we need a qualified ICE director now more than ever.”

John Sandweg, who served as acting ICE director under President Barack Obama, called Vitiello a “truly apolitical” career official who “worked his way up through sheer competence.”

“He’s not in any way, shape or form the kind of guy who would let political views shape his recommendation,” Sandweg said. “I’m sure Ron regrets doing anything on social media.”

In contrast to Homan, who had become a vocal proponent of Trump’s hard-line immigration agenda, including appearing at the lectern at White House news briefings, Vitiello has taken a lower-profile role since assuming command of ICE and its 20,000 employees.

But Vitiello came under criticism at his confirmation hearing earlier this month after he declined to rule out the possibility that the administration would again start separating migrant parents and children at the U.S.-Mexico border.

He said that detaining all the families, or splitting them up, could deter the rising number of migrants arriving at the border. But the zero-tolerance policy implemented earlier this year that caused the family separations sparked widespread public outrage, and the Trump administration stopped it.

Vitiello acknowledged the backlash, telling lawmakers during the hearing that the administration was not “ready for the public outcry that occurred.”

Regarding the Dennis the Menace tweet, the coalition of ICE unions said that “at best,” it showed “extremely poor judgment” on Vitiello’s part.

“Furthermore, we are concerned with what appears to be a political statement against the now President and the possibility that this tweet represents Mr. Vitiello’s true feelings about the President and his plans for the agency,” the unions wrote to Johnson and Sen. Claire McCaskill (Mo.), the committee’s ranking Democrat.

But Vitiello’s Twitter tendencies weren’t the only complaints from the ICE unions about him. Calling for a “change agent” in the job, the union leaders raised concern about his handling of the aftermath of a long protest at an ICE facility in Portland, Ore.

The union also told senators that Vitiello issued an order as acting director that barred some ICE employees from “going into their offices and performing their duties as law enforcement officers or law enforcement support staff.”

“The nominee to be considered by the Committee, Mr. Ronald D. Vitiello, has only been with ICE for a short period of time, and while this letter neither supports nor opposes his nomination to be our Director, we are aware of several matters that give us serious concern about him,” the unions said.

Sandweg said there has been a longtime rivalry between CBP, which deals with border arrests, and ICE, which handles deportations and investigations inside the country. Both agencies have been in the political crosshairs for years, with some prominent Democrats promoting calls to “abolish ICE” during the run-up to the midterm elections.

ICE has been accused in cases of abuse of immigrants, including the death in May — just before Vitiello took over — of a transgender woman from Honduras who died of dehydration and complications of HIV while in ICE custody. The woman, Roxsana Hernandez Rodriguez, 33, had traveled in a migrant caravan in the spring to reach the United States, and an independent autopsy concluded that she was physically abused, although ICE denied that it happened in the 16 days she was in the agency’s custody.

ICE has long been among the federal government sub-agencies with the lowest employee morale, ranking 288th of 339 departments in one survey from 2017, although that was a significant improvement from the previous year.

“What ICE needs is someone who puts their head down and does the job and lets the politics take care of itself,” Sandweg said. “Ron is as good a candidate as you could have hoped for.”

Johnson said after he delayed Vitiello’s committee vote that “we’re just trying to rapidly go through the issues raised by the union and do our due diligence to get answers about it.”

Johnson said the committee could vote as soon as Thursday on the nomination, which would then go to the floor for a final confirmation vote. The Senate Judiciary Committee would also separately vote on the nomination before it headed to the floor.

Seung Min KimSeung Min Kim is a White House reporter for The Washington Post, covering the Trump administration through the lens of Capitol Hill. Before joining The Washington Post in 2018, she spent more than eight years at Politico, primarily covering the Senate and immigration policy. Follow